This invention relates to an optoelectronic assembly and, more specifically, to a method and assembly for mounting active and passive optical devices and elements.
Optoelectronic components or active optical devices such as diode lasers, light-emitting diodes (LEDs) and photodiode detectors are used for printing, data storage, optical data transmission and reception, as well as pumping of high power lasers and a multitude of other applications. Optoelectronic packages are intended to provide a way for mounting passive and active optical elements and devices, as well as electrical components, in a robust structure which preserves proper alignment. Typically, an optoelectronic package includes an assembly upon which the optoelectronic components are mounted. The requirements of a package depend upon the application. In most cases, a package should provide precision alignment for the internal components, enable high speed electrical operation, provide for heat dissipation, match the coefficient of thermal expansion (CTE) between the mount and the device, and provide for simple external electrical connections and hermetic sealing. In addition, the package should be mechanically robust and be highly reliable. Clearly, satisfying all of these requirements calls for a judicious choice of materials and mounting techniques. In cases where numerous optical parts including other active devices and passive optical elements, e.g. lenses, gratings, fibers, mirrors and the like are intended to cooperate with each other, alignment of these parts with respect to each other is crucial.
These requirements have resulted in packages that are an order of magnitude larger, costlier, and more difficult to manufacture than purely electronic packages. In fact, the cost of most optoelectronic devices is dominated by the package rather than the optical devices themselves. New optoelectronic technologies will not succeed in the marketplace if the cost of packaging remains as high as it is now.
In a laser-fiber coupler, for example, the relative positions of fibers, lenses, mirrors and lasers must be precisely adjusted and permanently fixed to maintain beam coupling efficiency. Single-mode optical fibers, for example, require optical alignment tolerances of less than 1 xcexcm while multi-mode optical fibers require optical alignment tolerances of less than 10 xcexcm. To achieve precision alignment, the optoelectronic device is operated and monitored while the optical components are moved. The components are typically secured in place once a coarse alignment is achieved and then fine-tuned for optimal performance.
Precision alignment is complicated by the expansion and contraction of package materials during temperature fluctuations brought about primarily by the heat required to attach (solder) the individual active optical devices and variations in ambient temperature. Many prior art techniques use materials with varied coefficients of thermal expansion (CTEs). During thermal cycling the components of an optoelectronic device can drift out of alignment causing poor performance or even complete malfunction. Also, the mechanical stress produced can damage the components.
Existing packaging techniques often require that packages be manufactured individually. For example, individual constituent mechanical components of a package may be assembled into a finished device one at a time in an assembly line process. Batch processing techniques have been developed which can fabricate large numbers of optoelectronic assemblies. These techniques, however, are usually limited in their ability to manufacture a wide variety of optoelectronic devices and result in performance sacrifices. This is chiefly due to a high number of parts and a reliance on 3-D alignment. For example, conventional TO cans and high performance butterfly packages are not planar and thus their production cannot be easily automated.
The cost of present devices is further increased by the fact that optoelectronic assemblies frequently reside on substrates which need to be mounted on other substrates to produce the final package. For example, optoelectronic components mounted on silicon, a frequently-used material, must be remounted when placed in an optoelectronic package. Silicon is problematic for high-speed optoelectronic packages because it is a rather lossy dielectric. Progress in the fields of optics and electronics yields ever faster optoelectronic devices and therefore, this characteristic of silicon is limiting. Also, silicon is a brittle material susceptible to cracking and chipping, a liability in mechanically demanding applications. Further, its thermal conductivity is far lower than that of conventional heatsinks such as copper.
The teachings of U.S. Pat. No. 4,357,072, 4,119,363, and 4,233,619 hinge on proper placement and alignment of the optical and electronic components in three dimensions. Under these circumstances, the alignment of a fiber to a laser or the alignment of a lens to a detector is very difficult. Each component must be actively positioned, incurring all the expense associated with active alignment. Due to their design, these packages preclude the use of batch process manufacturing techniques. Also, high speed operation is problematic since these packages use numerous electrical connections and have complex geometries. Because the packages include dissimilar materials, great care is required to ensure that the differences in the CTEs of the materials do not cause misalignment during temperature fluctuations. This situation often leads package designers to compromise between heat dissipation and mechanical stability requirements.
Another disadvantage of these assemblies is the fact that they require a relatively large number of steps to fabricate. Each assembly has numerous subassemblies and, in general, one step is required for fabricating each subassembly. In turn, multiple steps and large numbers of subassemblies increase the cost, complexity and size of the package.
A technique for providing passive alignment between a plurality of diode lasers and optical fibers is described in U.S. Pat. No. 5,163,108. By forming alignment pedestals on the substrate for holding the laser chip and grooves in the substrate for holding optical fibers, simple alignment is accomplished. Unfortunately, this technique is limited because it does not provide an adaptable method for including other optical components such as lenses or mirrors. Further, the technique does not provide for enhanced heat dissipation for active elements generating large amounts of heat.
U.S. Pat. No. 5,123,074 describes a substrate for mounting active and passive optical elements and optical components. The substrate is made of an insulating block with metal regions formed for electrical connections and for mounting components. A reversed structure with a metal block and insulating regions is also described. Electrical circuits are formed on the insulating regions with the metal regions serving as bonding pads for the optical hardware. In the embodiment with an underlying metal block, the metal block acts as a ground plane, enhancing the high speed electrical characteristics of the substrate. The surfaces of the metal and insulating regions of the substrate lie in one plane, so that the substrate provides no mechanical alignment. Thus, this invention has the disadvantage of requiring the components to be actively aligned. Also, since the surface of the substrate is a single plane, different components cannot be mounted at different heights to allow for optical alignment.
U.S. Pat. No. 4,926,545 discloses a batch process for manufacturing optoelectronic assemblies. The assemblies can provide passive alignment or simplified active alignment for optoelectronic and optical components. The device includes metalization patterns for aiding alignment. The metalization patterns act as visual alignment aids for the placement of components. A problem with this device is that it uses silicon and therefore has all the disadvantages associated with silicon optoelectronic substrates. In addition, the process only provides for device positioning in which the optical axes are perpendicular to the substrate. This limits the ability of an optoelectronic circuit designer since orienting the optical axes parallel to the substrate allows one to design more complex optoelectronic circuits in a smaller space.
U.S. Pat. No. 5,119,448 discloses a method of making a substrate for mounting optical components by forming relief structures into the surface of the substrate. The relief structures provide mechanical alignment for the components. This method is primarily concerned with mounting fiber arrays and using such fiber arrays as sensors. No provisions are made for incorporating active optoelectronic components such as lasers and using the substrate as an optoelectronic assembly. Further, this method does not yield an optoelectronic package, or assembly for a package, and requires at least two patterning steps.
In the prior art, no single optoelectronic packaging technique offers the simultaneous advantages of high speed electrical operation, adaptability to produce various optoelectronic devices, adaptability to batch processing, effective heat dissipation and resistance to misalignment caused by changes in temperature. Yet a combination of these characteristics is very desirable for further progress in the field of optoelectronic packaging and circuit design.
In view of the above, it is an object of the present invention to provide an optoelectronic assembly which allows one to mount active optical devices, passive optical devices, and electronic components. The optoelectronic assembly may be used in optoelectronic packages and is compatible with any combination of active and passive optical devices and electronic components.
Another object of the invention is to provide a method for making the optoelectronic assembly. The method is particularly well-suited for batch processing fabrication. Additionally, the method is compatible with pick and place manufacturing techniques to achieve simple, precise, and reproducible active alignment of the active and passive devices and elements.
A further object of the invention is to provide an optoelectronic assembly which is compatible with high-speed electrical operation and which exhibits superior heat dissipation capability and mechanical stability over a wide temperature range.
Another object of the invention is to provide CTE matching between the optoelectronic components and the assembly. This increases device life and allows for a wider range of operating temperatures.
These and other objects and advantages will become apparent upon consideration of the ensuing description and the accompanying drawings.
These objects and advantages are achieved by an optoelectronic assembly having an insulating substrate with a planar surface and a metal layer bonded to the planar surface such that selected regions of the substrate are exposed and a step is produced between the substrate and a top surface of the metal layer. An active optical device such as a diode laser, optical amplifier, optical modulator, light detector or any other device is mounted on the metal layer. The active device has a first optical axis and in this arrangement this axis is parallel to the substrate. In addition, the metal layer provides an electrical path to the active device. It is essential for the optoelectronic assembly that the metal layer have a well-defined thickness. This thickness is selected such that the heat generated by the active device is dissipated, the substrate does not interfere with the propagation of light along the first optical axis, and such that the in-plane coefficient of thermal expansion (CTE) of the metal layer is constrained by the substrate.
Suitable thicknesses of the metal layer are between 50 xcexcm and 1,000 xcexcm. The metal layer is preferably made of a material selected from the group consisting of copper, copper-tungsten, copper alloys, copper composites, copper plated materials, copper clad materials, and copper laminates. The actual material selected will depend upon the purpose of the assembly and design specifications. The insulating substrate is thick enough to lend mechanical stability to the assembly, is a good dielectric for high speed electronics, and bonds well with metal. Alumina, BeO and AlN are good examples.
The optoelectronic assembly of the invention further admits a passive optical element with a second optical axis positioned in one of the selected regions. The positioning of this passive optical element should be such that the second optical axis is aligned with the first optical axis of the active device. Any passive elements such as optical fibers, lenses, filters, diffraction gratings, beam splitters, isolators and mirrors may be placed on the substrate.
In the preferred embodiment, the optoelectronic assembly incorporates square lenses and square isolators. An example of square lenses are graded index of refraction (GRIN) lenses, such as Gradium lenses. The metal layer is patterned as an electrical circuit and electrical components are mounted on the metal layer. This is particularly advantageous for highly integrated packages which include all three types of components, i.e. active and passive optical elements and electrical parts.
A bottom metal layer can be bonded to the insulating substrate opposite the planar surface. This second layer can also be patterned as an electrical circuit. In one embodiment the assembly with such a second layer has cooling elements mounted on the opposite side of the substrate. These cooling elements may include Peltier coolers. Micro-channel coolers using fluid coolants can also be patterned in the second layer of metal.
The method of making the optoelectronic assembly includes the steps of selecting the metal layer and substrate and bonding the metal layer to the substrate. In the patterning step, the metal layer is formed by photolithography, etching, stamping, electrodeposition or electro-discharge machining before bonding it to the substrate. Alternatively, photolithography or diamond is sawing is used to pattern the metal layer after the bonding step. The desired thickness is achieved by plating the metal layer on a thin metal film on the insulating substrate, or by a polish-back step. The active device may then be mounted on the metal layer.
During placing, the passive optical element is actively displaced in the selected region or is allowed to come to rest at the intended location under the force of gravity. In either case, the passive element moves along the step which serves as a mechanical fiduciary.
The method of the invention is particularly well-suited for batch processing. The assemblies are formed by patterning the metal layer on one large, flat insulating substrate which is then divided into at least two parts. Preferably, many assemblies are simultaneously produced. Bonding of the active devices on the metal layers can be performed before or after the dividing step. Likewise, the placement of passive elements can be performed before or after the dividing step, depending on the production process.
In still another embodiment, the active optical device is mounted on a submount and the step serves as a mechanical fiduciary to align the active optical device on this submount with any second active device or passive elements. The thickness of the submount is selected such that the optical axes of the components are aligned and the heat generated by the first active device is dissipated through the submount. A corresponding method is also a part of the invention.
The details of the invention will be better appreciated upon reading the detailed description below and referring to the drawing figures.